1. Field of the Invention
The present invention relates to an improved fuse tube for a cutout and, more particularly, to an improved fuse tube which exhibits improved operating performance. The improved fuse tube of the present invention may be used with a fuse link of the type described and claimed in commonly assigned co-filed United States Patent Application, Ser. No. 132,923 filed March 24, 1980 in the name of Richard J. Sabis. The fuse tube of the present invention may utilize the improved cutout described in commonly assigned, co-filed United States Patent Application, Ser. No. 132,924 filed March 24, 1980 in the name of Bruce A. Biller.
2. Discussion of the Prior Art
Single-vented fuse cutouts of various specific types are well known. A typical single-vented fuse cutout includes a hollow insulative fuse tube with a bore therethrough and conductive ferrules mounted to the opposite ends thereof. One ferrule (often called the "exhaust" ferrule) is located at an exhaust end of the bore and usually includes a trunnion casting which interfits with a trunnion pocket of a first contact assembly carried by one end of a porcelain or similar insulator. The other ferrule is normally held and latched by a second contact assembly carried by the other end of the porcelain insulator so that the fuse tube is normally parallel to, but spaced from, the porcelain insulator. The porcelain insulator is mountable to a cross-arm of a utility pole or a similar structure. A fuse link is located within the fuse tube bore with its ends respectively electrically continuous with the ferrules. One point of an electrical circuit is connected to the first contact assembly, while another point of the circuit is connected to the second contact assembly. Often, the insulator and the fuse tube are oriented generally perpendicular to the ground so that the exhaust ferrule and the first contact assembly are located below the other ferrule and the second contact assembly.
The fuse tube may include a high burst strength outer portion--for example, a fiber-glass-epoxy composite--lined with or containing an arc-extinguishing material, such as horn fiber, bone fiber, or vulcanized fiber. The arc-extinguishing material is ablative, that is, it decomposes into gaseous components when exposed to the heat of an electrical arc.
Normal currents in the electrical circuit flow without affecting the fuse link. Should a fault current or other over-current, to which the fuse link is designed to respond, occur in the circuit, the fuse link operates as described below. Operation of the fuse link permits the upper ferrule to disengage itself from the upper contact assembly, whereupon the fuse tube rotates downwardly due to coaction of the trunnion casting and the trunnion pocket. If the cutout operates properly, current in the circuit is interrupted and the downward rotation of the fuse tube gives a visual indication that the cutout has operated to protect the circuit.
Typical fuse links include a first terminal and a second terminal, between which there is normally connected a fusible element made of pure silver, silver-tin or the like. Also connected between the terminals may be a strain wire for a purpose described below. The second terminal is electrically continuous with, and is usually mechanically connected to, a button contact assembly, which is engageable by a portion of the upper ferrule on the fuse tube. The first terminal is connected to a flexible, stranded length of cable. Surrounding at least a portion of the second terminal, the fusible element, the strain wire (if used), the first terminal, and some portion of the flexible stranded cable is a sheath. The sheath is typically a cellulosic material impregnated with an ablative arc-extinguishing material (such as boric acid, magnesium borate, or the like) or may be made of an ablative arc-extinguishing material (such as horn fiber). Such ablative arc-extinguishing materials are well known and comprise compounds or compositions which, when exposed to the heat of a high-voltage arc, decompose to rapidly evolve large quantities of de-ionizing, turbulent and cooling gases. Typically, the sheath is much shorter than the fuse tube bore and terminates well short of the exhaust end thereof.
The free end of the stranded cable extends from the exhaust end of the bore and has tension or pulling force maintained thereon by a spring-loaded flipper on the trunnion casting. The tension or pulling force exerted on the cable by the flipper attempts to pull the cable and the first terminal out of the sheath and out of the fuse tube. The force of the flipper is normally restrained by the strain wire, many fusible elements not having sufficient mechanical strength to resist this tension or pulling force.
In the operation of typical cutouts, a fault current or other over-current results, first, in the melting or vaporization of the fusible element, followed by the melting or vaporization of the strain wire. Following such melting or vaporization, a high-voltage arc is established between the first and second terminals within the sheath, and the flipper is now free to pull the cable and the first terminal out of the sheath and, ultimately, out of the fuse tube. As the arc forms, the arc-extinguishing materials of the sheath decompose and high quantities of de-ionizing, turbulent and cooling gases are rapidly evolved. The movement of the first terminal under the action of the flipper, and the subsequent rapid movement thereof due to the evolved gases acting thereon as on a piston, result in elongation of the arc. The presence of the deionizing, turbulent and cooling gases, plus arc elongation, may, depending on the level of the fault current or other over-current, ultimately result in extinction of the arc and interruption of the current at a subsequent current zero. The loss of the tension on the stranded cable originally applied by the flipper permits the trunnion casting to experience some initial movement relative to the exhaust ferrule which, in turn, permits the upper ferrule to disengage itself from the upper contact assembly. This initiates the downward rotation of the fuse tube and its upper ferrule to a so-called "drop out" or "drop down" position.
As noted immediately above, arc elongation within the sheath and the action of the evolved gases may extinguish the arc. At very high fault current or over-current levels, however, arc elongation and the sheath may not, by themselves, be sufficient to achieve this end. Simply stated, at very high fault current levels, either the sheath may burst (because of the very high pressure of the evolved gas therewithin) or insufficient gas may be evolved therefrom to quench the high current level arc. For these reasons, the fuse tube is made of, or is lined with, ablative arc-extinguishing horn fiber, bone fiber or vulcanized fiber, as noted above. In the event the sheath bursts, the arc-extinguishing material of the fuse tube interacts with the arc; gas evolved as a result thereof effects arc extinction. If the sheath does not burst, the arc-extinguishing material of the fuse tube between the end of the sheath and the exhaust end of the fuse tube bore is nevertheless available for evolving gas in addition to that evolved from the sheath. The joint action of the two quantities of evolved gas, together with arc elongation, extinguishes the arc.
Typically, the fuse tube bore has a circular cross section and the portion thereof closer to the upper ferrule is just large enough to accommodate the insertion thereinto of the fuse link and, specifically, of the sheath thereof. In typical fuse cutouts, placement of the fuse link in the fuse tube bore closes the end thereof near the upper ferrule but the exhaust end of the bore remains open. As noted earlier, it is through this exhaust or open end of the bore that the cable of the fuse link extends.
Improper operation, or lack of operation, of fuse cutouts and typical fuse tubes thereof, as described above, have been detected. Specifically, at or near the maximum interrupting current rating of the above-described cutouts, improper current interruption or failure to interrupt current has been detected. An examinaion of typical cutouts and their fuse tubes, both during and after attempts at operation, has led to the conclusion that gas evolved deep in the bore--that is, remote from the exhaust end and whether evolved from the sheath or from the walls of the bore itself--often stagnates, that is, is prevented from efficiently exiting from the exhaust end of the bore due to the pressure of gas evolved from the bore in the vicinity of the exhaust end. Such stagnation may be referred to as "clogging" of the bore. More specifically, because at high interrupting current levels arcing starts deep in the bore and, indeed, within the sheath, and because at high current levels arcing often continues as the first terminal of the fuse link nears the exhaust end of the bore, the gas evolved deep within the bore is often prevented from freely exiting the exhaust end thereof due to the pressure generated by gas evolved by the wall of the bore close to the exhaust end. It has also been observed that as the first terminal of the fuse link nears the exhaust end of the bore it partially blocks the exhaust end; this partial blockage adds to the stagnation of gas evolved deep within the bore (clogging). It has been postulated that the stagnation of gas evolved deep within the bore (clogging), due to arcing before a current zero occurs, prevents recovery of sufficient dielectric strength within the bore at the current zero, thus preventing effective and permanent current interruption.
The general object of the present invention, then, is to improve the fuse tube of typical fuse cutouts to eliminate the above-described problems and to improve and render more efficient the operation thereof.